This invention relates in general to surface acoustic wave sensors and in particular to a surface acoustic wave sensor for a liquid sensor application that utilizes a shear horizontal or quasi shear horizontal mode of wave propagation along a predetermined axis.
Surface Acoustic Wave (SAW) devices are electronic components that generate guided acoustic waves along a surface of the device. SAW devices are generally fabricated on single crystal anisotropic substrates that are also piezoelectric. SAW devices typically include one or more pairs of intertwined interdigital electrodes that form transducers to convert the electrical signals applied to the device into the electromechanical surface acoustic waves generated in the device. The devices also may have one or more thin film layers deposited upon the substrate surface.
SAW sensors are among the most sensitive and widely used physical and chemical sensors in gas and liquid environments because the propagating acoustic wave has its energy concentrated close to the device surface. Along an arbitrary surface wave propagation direction, a particle in the substrate material describes an elliptical trajectory, with displacement components normal and parallel to the device surface. For a liquid sensor application, any SAW device operational mode with a significant particle displacement component normal to the surface suffers severe attenuation, thus compromising the device performance. Accordingly, for a liquid sensor, it is desirable that the selected operational mode presents a high or exclusive particle displacement component parallel to the substrate surface, since this acoustic mode is less attenuated by the presence of the liquid than in the case of particle displacement that is normal to the substrate surface.
A SAW that satisfies the above described particle displacement condition is the Shear Horizontal (SH) wave, also known as a Surface Transverse Wave (STW). Pure, piezoelectrically active, SH waves occur along propagation directions in which the sagittal purely mechanical displacement components are uncoupled from the electrical fields of the device and the shear horizontal displacement components of the waves, leading to two separate solutions along those propagation directions. One solution is a purely mechanical sagittal SAW while the other solution is a stiffened shear horizontal wave mode that is generated by the interdigital transducers described above. Both SAW solutions can exist on rotated Y cuts of trigonal class 32 crystals.
One trigonal class 32 crystal is quartz. However, with respect to SAW devices to be utilized as liquid sensors, use of SH-SAW on quartz crystals for liquid sensing applications poses a problem in that the effective permittivity of quartz is around 4.6, and thus the interdigital transducers are electrically shorted by the presence of high relative permittivity fluids, such as water, which has a relative permittivity around 80.
The trigonal class 32 crystals also includes the LGX family of crystals, which comprise langatate (LGT, La3Ga5.5Ta0.5O14), langasite (LGS, La3Ga5SiO14), langanite (LGN, La3Ga5.5Nb0.5O14), and variations, such as LGTS La3Ga5.25Ta0.25Si0.5O14) and LGZS (La3Ga5Zr0.5Si0.5O14). While the LGX family of crystals can also present SH-SAW, for liquid sensing applications, theoretical predictions and experimental verification for crystals of the LGX family along known propagation directions defined by Euler angles (0°, 70°, 90°) and (0°, 132°, 90°), have shown that these orientations have a high penetration depth inside the surface. The high penetration depth translates into a weakly surface guided wave, with the wave energy penetrating significantly inside the substrate. Accordingly, the sensitivity to any surface perturbation, a necessary mechanism for a liquid sensor application, is reduced for known sensors using the LGX family of crystals along the orientations with the above Euler angles.
A Pseudo Surface Acoustic Wave (PSAW) is another known type of acoustic wave mode that has been used for liquid sensing, in particular along 36° Y rotated X propagation with a substrate formed from LiTaO3 material, which is referred to as a 36° Y SAW device in the following descriptions. However, disadvantages of this mode with respect to the SH-SAW are the fact that the PSAW is not strictly guided, and therefore a spurious Bulk Acoustic Wave (BAW) is generated in the device, increasing the losses, and ultimately degrading the performance.
Accordingly, it would be desirable to provide a SH-SAW sensor that could be utilized as a liquid sensor.